Wind turbines can encounter excess wind speeds beyond their structural design capacity or beyond what is needed for maximum rated power output. The “rated wind speed” of a wind turbine is the lowest wind speed at which it produces power at its rated capacity. Damaging blade loads can be sustained above this rated wind speed in what is called the ‘post-rated’ regime, typically 12-25 m/s or 15-20 m/s for example, depending on the turbine. To maximize efficiency before reaching rated power and to regulate power to a fixed level above rated wind speed, the turbine varies the blade pitch depending on torque/power output. Beyond the safe operating wind speed, the blades are feathered, and the rotor may be stopped or idled.
To maintain power production in the post-rated wind speed regime, the blade is pitched progressively towards feather with increasing wind speed. As a result, the outboard parts of the blade may have a near zero or negative angle of attack, at which they are at or near idle or are in a negative lift or “braking” state. In such conditions, most or all of the rotor torque is generated by inboard parts of the blade (FIG. 1).
When operating at rated wind speed or less, the blade pitch is set for an angle of attack that maximizes power production and is associated with lift being generated at all blade radial positions. A sudden gust increases the angle of attack to a stall condition that reduces lift temporarily, thus protecting the blade from lift overloads and rapid load changes. However in the post-rated wind configuration with zero or negative angle of attack on parts of the blade, this natural load alleviation mechanism is reduced because the outboard blade sections are operating far from stall. In this configuration, a gust initially increases the angle of attack toward maximum lift, causing a large and rapid load increase on the outer portion of the blade, leading to higher amplitude fatigue load cycles. FIG. 2 illustrates that a relatively small increase in lift coefficient 25 causes stalling in normal operation compared to a larger increase 26 needed to stall at a low angle of attack as conventionally used in high winds.